With the aim of improving both the luminous power and the energy yield of vehicle lighting devices, in particular headlamps, there is a current technical trend to replace filament lamps with gas discharge lamps. Unlike conventional bulbs, which were designed to be connected directly to the battery of the vehicle, these new lamps require high voltages, AC or DC depending on the operating mode, in order to create and maintain the electrical discharge in the gas.
These high voltages, which are specific to each type of lamp (around 25 kV for igniting a xenon lamp), are produced from the on-board voltage by a supply module which also regulates the power and is known under the name of “ballast”.
This supply module generally comprises, in a manner known per se, a DC/DC converter for producing a high DC voltage from the low voltage of the battery, a DC/AC converter for producing the AC voltage necessary for supplying the lamp, and a circuit for producing the start-up pulse.
These various blocks are controlled by an electronic control block which regulates the power supplied to the lamp as a function of its current/voltage characteristic, which varies according to the operating phases and evolves over the service life of the lamp.
This is therefore an intrinsic power regulation associated with the operation of the lamp itself
By contrast, the development of evolved or advanced lighting systems of the AFS type (Advanced Front lighting System or Adaptive Front lighting System) for motor vehicles has made it apparent that there is a need for regulation and/or variability of the power from outside the ballast.
The new dynamic lighting technology DBL (Dynamic Bending Light) adapts the orientation of the front headlamps as a function of the speed of the vehicle and the bends in the road.
Advanced lighting systems also take account of the ambient light at night, and during the day are capable of turning the headlights on and off when entering and leaving a tunnel.
In simple manual-control systems, the intensity of the headlights must also be able to change rapidly: switching between full beam/dipped beam, flashing headlights.
In this latter case, the lamp must work for a short time beyond its nominal power; in other circumstances, it is not necessary for it to continue to operate at its nominal power.
It has been found that a precise adaptation of the electrical power supplied to the lamp as a function of requirements leads, on average, to a reduction in the consumption of on-board energy, and to an improvement in the service life of the lamp.
Systems for managing the applied electrical power are known in many technical fields in which gas discharge lamps are used.
However, the technical specifications of a method and a device for managing a ballast intended for motor vehicles comprise specific constraints which generally do not exist in other applications:                speed of passing from one operating mode to another;        ability to remotely signal faults (breakdown diagnostics);        ability to be installed on board;        compatibility with existing electronic equipment, in particular the standard on-board networks, for example LIN (Local Interconnect Network) or CAN (Controller Area Network).        
In the field of equipment for motor vehicles, there is specifically known a ballast for a discharge lamp in which the power delivered is controlled by an external signal. This ballast is described in U.S. patent application U.S. Pat. No. 6,051,939, which was published on 18 Apr. 2000.
This device is based essentially on the storage and use of the current/voltage characteristic of the lamp. It therefore concerns a method for externally controlling the power of the lamp at any given time, rather than a method for selecting the operating mode with a view to optimizing the service life.
On the other hand, such a method in which, during the ignition phase, the intensity of the current and/or of the supply power of the lamp is controlled as a function of a parameter representative of the external luminosity of the vehicle, in order to effectively optimize the service life of the lamp, is described for example in European patent application EP1248498 by the Applicant, which was published on 9 Oct. 2002.
However, the scope of this latter method is restricted to the particular case of controlling the headlights as a function of the ambient lighting.
As shown in the prior art documents mentioned above, no generic method or device exists for managing a discharge lamp ballast in order to optimize the service life of the lamp, the method or device being adapted to the various present or future intelligent lighting systems for motor vehicles.